[R] with vs. attach

[peter dalgaard]

On 06 May 2016, at 02:43 , David Winsemius <dwinsemius at comcast.net> wrote:

> 
>> On May 5, 2016, at 5:12 PM, Spencer Graves <spencer.graves at effectivedefense.org> wrote:
>> 
>> I want a function to evaluate one argument
>> in the environment of a data.frame supplied
>> as another argument.  "attach" works for
>> this, but "with" does not.  Is there a way
>> to make "with" work?  I'd rather not attach
>> the data.frame.
>> 
>> 
>> With the following two functions "eval.w.attach"
>> works but "eval.w.with" fails:
>> 
>> 
>> dat <- data.frame(a=1:2)
>> eval.w.attach <- function(x, dat){
>>  attach(dat)
>>  X <- x
>>  detach()
>>  X
>> }
>> 
>> eval.w.with <- function(x, dat){
>>  with(dat, x)
>> }
>> 
>> eval.w.attach(a/2, dat) # returns c(.5, 1)
> 
> How about using eval( substitute( ...))?
> 
> eval.w.sub <- function(expr, datt){
>   eval( substitute(expr), env=datt)
>                         }
> eval.w.sub(a/2, dat)
> #[1] 0.5 1.0
> 
> 

Actually, I think a better overall strategy is to say that if you want to pass an expression to a function, then pass an expression object (or a call object or maybe a formula object). 

Once you figure out _how_ your eval.w.attach works (sort of), you'll get the creeps: 

Lazy evaluation causes the argument x to be evaluated after the attach(), hence the evaluation environment of an actual argument is being temporarily modified from inside a function. 

Apart from upsetting computer science purists, there could be hidden problems: One major issue is that  values in "dat" could be masked by values in the global environment, another issue is that an error in evaluating the expression will leave dat attached. So at a minimum, you need to recode using on.exit() magic.

So my preferences go along these lines:

> dat <- data.frame(a=1:2)
> eval.expression <- function(e, dat) eval(e, dat)
> eval.expression(quote(a/2), dat)
[1] 0.5 1.0
> eval.expression(expression(a/2), dat)
[1] 0.5 1.0

> eval.formula <- function(f, dat) eval(f[[2]], dat)
> eval.formula(~a/2, dat)
[1] 0.5 1.0

Peter D.



> -- 
> David.
> 
> 
>> 
>> eval.w.with(a/2, dat) # Error ... 'a' not found
>> 
>> 
>> Thanks, Spencer Graves
>> 
>> 	[[alternative HTML version deleted]]
>> 
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> 
> David Winsemius
> Alameda, CA, USA
> 
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

-- 
Peter Dalgaard, Professor,
Center for Statistics, Copenhagen Business School
Solbjerg Plads 3, 2000 Frederiksberg, Denmark
Phone: (+45)38153501
Office: A 4.23
Email: pd.mes at cbs.dk  Priv: PDalgd at gmail.com